The present invention relates generally to methods and apparatus for attaching a sensor to a tubing string for deployment within a wellbore. More specifically, the present invention relates to methods and apparatus for attaching a sensor to a tubing string for deployment within a highly deviated wellbore.
During the production of hydrocarbons from an underground reservoir or formation, it is important to determine the development and behavior of the reservoir and to foresee changes which will affect the reservoir. Methods and apparatus for determining and measuring downhole parameters for forecasting the behavior of the reservoir are well known in the art.
A standard method and apparatus includes placing one or more sensors downhole adjacent the reservoir and recording seismic signals generated from a source often located at the surface. Hydrophones, geophones, and accelerometers are three types of sensors used for recording such seismic signals. Hydrophones respond to pressure changes in a fluid excited by seismic waves, and consequently must be in contact with the fluid to function. Seismic waves are waves of elastic energy, approximately in the range of 1 to 100 Hz, having both a compressional and a shear component, where the compressional component, or P-wave, oscillates in a direction parallel to propagation of the wave, and the shear component, or S-wave, oscillates in a direction perpendicular to the propagation of the wave.
Hydrophones are non-directional and respond only to the compressional component of the seismic wave. They can be used to indirectly measure the shear wave component when the shear component is converted to a compressional wave (e.g. at formation interfaces or at the wellbore-formation interface). Geophones measure both compressional and shear waves directly They include particle velocity detectors and typically provide three-component velocity measurement. Accelerometers also directly measure both compressional and shear waves, but instead of detecting particle velocities, accelerometers detect accelerations, and hence have higher sensitivities at higher frequencies. Accelerometers are also available having three-component acceleration measurements. Both geophones and accelerometers can be used to determine the direction of arrival of the transmitted waves.
Other sensors are available that enable various parameters to be measured, especially acoustic noise, natural radioactivity, temperature, pressure, etc. The sensors may be positioned inside the production tubing for carrying out localized measurements of the nearby annulus or for monitoring fluid flowing through the production tubing. While the location within the wellbore of some of these sensors is not critical, in the case of geophones and accelerometers, the sensors must be mechanically coupled to the formation in order to conduct the desired measurement.
One method of coupling a sensor to the formation is by providing the wireline sonde with a mechanical arm which can be extended against the wall of the casing. The arm may be extended by mechanical means, fluid pressure, or electrical actuation. When extended, the arm presses the sensor against the opposite wall of the casing with a force sufficient to prevent relative motion of the sensor with respect to the casing. As a rule of thumb, the force applied by the arm should be at least five times the weight of the sensor, and it is not uncommon for sensors to weigh 30 lbs. or more.
Another mechanism for coupling a sensor to a formation involves the use of springs to force the sensor against the wall of the casing. The sensor is maintained in a retracted position while the tubing string is run into the wellbore. When the tubing string has reached its deployment location, the springs are released and force the sensor against the casing. As in those designs employing arms, the springs are designed to provide a certain force to push the sensor onto the casing. When operating in highly deviated well sections, including near horizontal and horizontal sections, both spring and arm systems have faced challenges. In contrast to a normal vertical wellbore, where the string is likely to be somewhat centered, in these highly deviated sections the tubing string is likely to rest against the casing with some or all of the weight of the string bearing against the casing wall.
Although most spring and arm systems are designed to actuate with a force greater than the weight of the sensor, they may not have enough force to push the string away from the casing, when the sensor is between the casing and the tubing, or sufficient reach to push the sensor against the far wall of the casing, when the tubing is directly against the casing. If the system fails to fully actuate, the sensor may not be maintained in the desired, stable relationship with the wellbore, making data acquisition conditions less than ideal.
Thus, there remains a need in the art for methods and apparatus to deploy sensors into highly deviated sections of a wellbore. Therefore, the embodiments of the present invention are directed to methods and apparatus, for attaching a sensor to a tubing string for deployment in a highly deviated wellbore section, that seek to overcome these and other limitations of the prior art.